This invention relates generally to an electrostatographic printing machine, and more particularly concerns a printing machine adapted to operate either in a pictorial or functional copying mode.
In a typical electrostatographic printing machine, a latent image is recorded on a surface and developed with charged particles. A sheet of support material is positioned closely adjacent to the latent image and arranged to have the particles transferred thereto. After the particles are transferred to the sheet of support material, they are permanently affixed thereto forming a copy of the original document. Electrophotographic printing and electrographic printing are different versions of electrostatographic printing. In the process of electrophotographic printing, a photoconductive member is charged to a substantially uniform level. The light image of an original document irradiates the charged photoconductive member dissipating the charge in accordance with the intensity thereof. This records an electrostatic latent image on the photoconductive member corresponding to the original document being reproduced. Electrographic printing differs from electrophotographic printing in that neither a photoconductive member nor a light image is required to create a latent image of the original document. Generally, both of the foregoing processes employ heat settable particles to develop the latent image. Heat is applied to these particles permanently affixing them to the sheet of support material.
A multi-color electrophotographic printing machine is essentially the same as a black and white printing machine. However, in multi-color printing, the foregoing process is repeated a plurality of cycles, each cycle being for a discrete color. In this process, the light image is filtered to record an electrostatic latent image on the photoconductive member corresponding to a single color. A plurality of different single color light images are formed. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. The toner powder images are then transferred to the sheet of support material in superimposed registration with one another. The multi-layered toner powder image is then permanently affixed by the application of heat thereto to the sheet of support material producing a permanently colored copy of the original document.
Heretofore, an electrophotographic printing machine was merely adapted to produce either a functional copy or a pictorial copy. A functional copy is a copy of a document wherein subtle variations of tone or color are not present, such as in a graph, chart, lines, etc. Functional copying machines have great difficulty in forming tone gradations. Contrawise, a pictorial copying machine employs a half-tone screen to overcome this defect. The screen produces tonal gradations by forming half-tone dots or lines of varying size. In the highlight regions, the half-tone pattern may comprise narrow lines or small dots. The lines increase in width or the dots in size throughout the intermediate shades until they merge together at the shadow end. In this manner, there will be complete whiteness at the highlight end and nearly solid black at the shadow end of the tone scale.
Numerous patents teach the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,493,381; 3,776,633; and 3,809,555. A recent Japanese application shows the utilization of an auxiliary light source in conjunction with a screen in the optical light path. This is Japanese application No. 47-124,202 dated Dec. 11, 1972 having a laid open Japanese application No. 49-82345, the inventor being Masayasu Anzai and the applicant being Hitachi, Ltd. Other copending applications relevant thereto are Ser. No. 511,976, filed Oct. 4, 1974, and now U.S. Pat. No. 3,936,173, and Ser. No. 507,169, filed Sept. 18, 1974.
However, none of the prior art references appear to disclose an electrophotographic printing machine having the capability of operating in either the functional or pictorial copying mode. Nor do the prior art references appear to teach electrophotographic printing machines having the ability to vary the reproduction contrast of copies by adjusting the half-tone screen. Thus, no printing machine hereinbefore developed had the capability of de-energizing the screen selectively so as to change from the pictorial mode to the functional mode.
Accordingly, it is a primary object of the present invention to improve an electrostatographic printing machines by providing simple contrast adjustments as well as pictorial and functional copying modes.